Extractive distillation is a key separation method used in chemical processing, especially for the recovery of aromatics from hydrocarbon mixtures. A key aspect of extractive distillation is that a separating agent, a solvent with a high boiling point, is added to a component mixture for separation which increases relative volatility when the components have similar or close boiling points.
Aromatic hydrocarbons can be recovered from mixtures containing aromatic and non-aromatic hydrocarbons by liquid-liquid extraction. Examples of suitable feedstocks for extraction are mixtures containing benzene, toluene and xylene derived from the catalytic reforming of naphtha or from the hydrogenation of raw pyrolysis gasoline byproduct from olefin production.
Extractive distillation is an alternative aromatics-recovery process which typically is applied to lighter hydrocarbon fractions and comprises an extractive-distillation column and a solvent-recovery column. A nonvolatile solvent passes to an upper section and the hydrocarbon fraction is introduced to a middle section of the extractive-distillation column. As the solvent descends through the column, it preferentially extracts the polar components (aromatics) to form a rich solvent while the non-polar component vapor comprising non-polar components ascends to the top of the column. Overhead vapor is condensed, with a portion of the condensate being recycled to the top section of the extractive-distillation column as reflux while a net portion is withdrawn as a raffinate product. The rich solvent from the bottom of the extractive-distillation column, comprising the solvent and the polar components, is fed into a solvent recovery column to recover aromatics as an overhead product along with reflux to the recovery column. Bottoms from the solvent-recovery column are lean solvent, free of the feed components, which is recycled to the upper portion of the extractive-distillation column as the extractive solvent.
Extractive distillation requires less equipment, for example two instead of four separation columns, and has a lower energy requirement than solvent extraction, but the application of this process is restricted by requiring a narrower feedstock boiling range than liquid-liquid extraction. The feedstock to extractive distillation normally is obtained by fractionation of a wider-range hydrocarbon fraction. Extractive distillation most often is targeted to the recovery of benzene from a fraction rich in C6 and C7 hydrocarbons, but small amounts of heavy hydrocarbons remain even in such light fractions. These heavy hydrocarbons tend to remain with the rich solvent at the bottom of the extractive-distillation column due to their high boiling points, and accumulate in the solvent due to the solvent being circulated within a closed loop, with consequent inefficient column and process operation. These generally can only be removed from the solvent by increasing the temperature, vacuum level, and stripping steam of the solvent-recovery column, but this method is costly and may result in degradation of the solvent. There is a need for an efficient process to remove heavy hydrocarbons and products of degradation from the circulating solvent to reduce sludge and plugging in the system.
Solvent purification in a liquid-liquid extraction process is disclosed in U.S. Pat. No. 4,048,062 to G. Asselin, in which a portion of lean solvent is introduced into a solvent regeneration zone and stripped with steam to remove deteriorated solvent and impurities. Extractive distillation is widely referenced in patent and other literature. The process is generally described in F. Lee, et al., “Two Liquid-Phase Extractive Distillation for Aromatics Recovery”, Ind. Eng. Chem. Res. (26) No. 3, 564-573, 1987. U.S. Patent Applications 2009/0038991 and 2010/0300939 disclose extractive distillation processes in which the solvent is subjected to washing and recovery to separate heavy hydrocarbons and sludge and avoid accumulation of hydrocarbons heavier than the desired product; relevant provisions of these applications are incorporated herein by reference thereto.